THE SYNTHESIS OF NEW SUBSTANCES 37I 



the microsomes. We shall see that one of the main theories concerning the 

 chemical processes of development attributes great importance to the 

 microsomes in the synthesis of cell-specific proteins. Their condition in 

 the undamaged or uncentrifuged cell is still uncertain, since they range 

 down to a size which is too small to be observed in living material. It is 

 possible that in hfe they do not exist as separate particles, but that the 

 'microsomes' fotmd after high-speed centrifugation are really the products 

 of the breakdown of the membranous structures of the clear cytoplasm 

 which are seen in Fig. 17.2. 



In embryonic cells there are often many other granules which do not 

 fall quite clearly into the two classes of mitochondria and microsomes 

 (cf. Holtfreter 1946). As has been pointed out (p. 41), several different 

 kinds of granule may be built in to the cytoplasm of the developing 

 oocyte, coming either from the nurse cells or the germinal vesicle or 

 arising in situ. It seems probable that during the early stages of develop- 

 ment some of these are being gradually transformed into the typical forms 

 of mitochondria and microsomes. Before this transformation is complete, 

 they are referred to either by special names specially invented to cover 

 particular cases (e.g. 'a-granules' and similar phrases) or by general terms 

 such as 'hpochondria', etc. It is possible, also, that the microsomes may 

 gradually develop into mitochondia (Brachet 1952). 



2. Arguing from the gene to the substance 



There are two main methods of approach to the problem of the pro- 

 duction of substances by genes; one which starts from the genes and at- 

 tempts to link them up with substances which can be identified as their 

 immediate products, the other which starts from substances which are 

 known to be under genetical control and tries to delve behind their 

 antecedents until it reaches the genes. We shall consider the former 

 approach first; it has so far not proved very productive. 



Logically our first step should be to defme the gene. Nowadays it is 

 not easy to do this in an unequivocal fashion. Originally, a MendeHan 

 factor was an abstract entity invoked to explain the numerical relations 

 among the offspring of crosses. Then it was discovered that the factors 

 are carried by chromosomes and the term 'gene' was introduced to refer 

 to the physical entity which constitutes a unit factor. The difficulty is to 

 know how to distinguish one unit factor from another. There are several 

 possible criteria. The best known is based on crossing-over. Two genes 

 are considered different if crossing-over can take place between them. 

 One difficulty with this definition is that certain regions of chromosome 

 are known (e.g. the Y chromosome in Drosophilia or the central parts of 



